The present invention relates to a catalyst for reducing nitrogen oxides in oxidizing and reducing atmospheres. The catalyst contains iridium on a support material.
In a similar manner to diesel engines, it is now being attempted to lower the fuel consumption of modern gasoline engines by also operating them with lean air/fuel mixtures. Fuel savings of up to 25% are expected from so-called lean-burn engines, in particular those with direct gasoline injection, as compared with stoichiometrically operated internal combustion engines. However, lean-burn engines also have operating phases with stoichiometric or so-called rich air/fuel conditions. These types of conditions prevail after a cold-start, when accelerating and when under full load. Diesel engines, which are operated almost exclusively with lean air/fuel mixtures, also belong to the class of lean-burn internal combustion engines.
The catalytic removal of nitrogen oxides contained in the exhaust gas is a substantial problem in the case of lean-burn engines. Due to the high oxygen concentration in the exhaust gas from these engines, up to 15 vol. %, the nitrogen oxides (NOx) contained in the exhaust gas cannot readily be reacted with the hydrocarbons (HC) and carbon monoxide (CO) also contained in a lean exhaust gas on a conventional exhaust gas catalyst, because in this case the reductive components (HC and CO and also small amounts of hydrogen H2) are oxidized directly by oxygen.
Exhaust gas catalysts for the simultaneous conversion of hydrocarbons, carbon monoxide and nitrogen oxides, so-called three-way catalysts, require a stoichiometric composition of exhaust gas, with an oxygen concentration of about 0.7 vol. %, for the conversion to take place. The exhaust gas composition is usually described by the normalized air to fuel ratio xcex, which is defined as the air/fuel ratio normalized to stoichiometric conditions. The air/fuel ratio states how may kilograms of air are required for complete combustion of one kilogram of fuel. With conventional fuels, the stoichiometric air/fuel ratio has a value of 14.6, which corresponds to a normalized air/fuel ratio of 1.
Two alternative routes have been described for converting nitrogen oxides in lean exhaust gases. An attempt is made to store the nitrogen oxides in the form of nitrates during lean operation of the internal combustion engine, with the aid of so-called nitrogen oxide storage catalysts. Preferred storage materials for this purpose are, for example, alkaline earth metal oxides, in particular barium oxide. For storage purposes, the nitrogen oxides, between 50 and 90 vol. % of which consists of nitrogen monoxide depending on the type of engine and mode of operation of the engine, first have to be oxidized to nitrogen dioxide before they can form nitrates with the storage materials. Oxidation takes place mainly on the storage catalyst itself and this is provided, for example, with platinum as a catalytically active component for this purpose.
Depending on the driving conditions, the storage material has to be regenerated at certain intervals. For this, the internal combustion engines are operated for brief periods with rich air/fuel mixtures. Under the reductive exhaust gas conditions which then prevail, the nitrates are decomposed and the nitrogen oxides being released are converted into nitrogen, with simultaneous oxidation of the reductive components. The acceleration phases may sometimes be used for regeneration of the storage material. In addition, however, in the absence of acceleration phases, targeted regeneration is required and this has to be achieved by appropriate regulation of the engine. The fuel required for this reduces the theoretical fuel saving when using lean-burn engines.
Current storage catalysts still exhibit high sensitivity towards sulfur oxides contained in the exhaust gas from internal combustion engines. Sulfur oxides, after oxidation to sulfur trioxide on the storage catalyst, react with the storage material to form thermally very stable sulfates and continuously reduce the storage capacity for nitrogen oxides.
As an alternative to nitrogen oxide storage catalysts, catalysts have been developed which have a higher selectivity than conventional catalysts during the reaction of nitrogen oxides with hydrocarbons in an oxygen-rich exhaust gas. These include, for example, catalysts based on zeolites exchanged with copper or iron or iridium-containing catalysts. These catalysts enable permanent conversion of nitrogen oxides even in lean exhaust gases.
The activity of reduction catalysts generally depends on the oxygen concentration of the exhaust gas and on the temperature of the exhaust gas. Thus, Chajar et al. reported, in Catalysis Letters 28 (1994), 33-40, that a Cu-ZSM5 catalyst displays its optimum reduction activity with about 0.5 vol. % of oxygen in the exhaust gas, that is under slightly sub-stoichiometric conditions. If there is no oxygen in the exhaust gas, the conversion of NO on this catalyst is between 2% (at 250xc2x0 C.) and 8% (at 500xc2x0 C.), depending on the temperature of the exhaust gas.
In addition to depending on the oxygen concentration of the exhaust gas, reduction catalysts also exhibit a pronounced temperature dependence with regard to the conversion of nitrogen oxides. The light-off temperature for the reaction of nitrogen oxides in an oxygen-rich exhaust gas is about 350xc2x0 C. The light-off temperature is understood to be the temperature at which the rate of conversion of a harmful substance reaches a specific value, usually 50%. As the exhaust gas temperature increases beyond this point, the conversion rate for nitrogen oxides initially increases, passes through a maximum at a specific temperature and then decreases again to almost zero at exhaust gas temperatures above 500xc2x0 C.
Lean-burn gasoline engines, and in particular diesel engines, often achieve exhaust gas temperatures of less than 350xc2x0 C. when operating under part loads. Therefore catalysts are required which develop their maximum rates of conversion at the lowest possible exhaust gas temperatures of less than 350xc2x0 C., preferably less than 300xc2x0 C.
EP 0 633 052 B1 describes a catalyst for the conversion of nitrogen oxides in oxygen-rich exhaust gases which consist of a crystalline iridium silicate with a Si/Ir atomic ratio of 50 to 800 and a Si/Al ratio of not less than 15. With an oxygen concentration of 3.5 vol. % in the exhaust gas, the maximum rates of conversion for this catalyst occur at exhaust gas temperatures of at least 430xc2x0 C. and thus are not very suitable for the case described above. As a result of the method of preparation chosen for this catalyst, a defined compound of silicate and iridium is present, which leads to very homogeneous and atomic distribution of the iridium in this compound.
EP 0 832 688 A1 describes a catalyst which contains iridium, sulfur and optionally platinum as catalytically active substances. In this catalyst, iridium and sulfur can be applied to a common support material such as, for example, aluminum oxide. Alternatively, a metal sulfate may also be used as a support for the iridium. After impregnating the support material with iridium chloride, the material is dried and calcined at 500xc2x0 C., so that the iridium is present as very fine particles on the support material. The catalyst is used to remove nitrogen oxides from oxidizing exhaust gases.
DE 196 19 791 A1 describes a catalyst which contains iridium, an alkali metal and at least one metal carbide and/or metal nitride as support. In that document, iridium and the alkali metal are applied to the support, for example, by simultaneous impregnation of the support material with soluble precursor compounds of iridium and the alkali metal. With an air/fuel ratio of 23, the temperature for maximum conversion of nitrogen oxides with this catalyst is about 350xc2x0 C.
JP 07080315 A1 also discloses a catalyst for removing nitrogen oxides from oxidizing exhaust gases from lean-burn engines and diesel engines. The catalyst contains iridium as active component on a support material. The support materials used include, inter alia, silicon dioxide and X, Y, A, ZSM-5 zeolites, mordenite and sillimanite.
An object of the present invention is to provide a catalyst for the reduction of nitrogen oxides which is distinguished by a maximum for the conversion rate at low exhaust gas temperatures and which also has exceptional resistance to poisoning by sulfur dioxide contained in the exhaust gas.
A further object of the invention is to enable a catalyst to withstand the varying conditions present in a lean-burn engine and to have sufficiently high activity for the reduction of nitrogen oxides both under lean and under rich operation.
The above and other objects of the invention can be achieved by a catalyst for the reduction of nitrogen oxides in oxidizing and reducing atmospheres which contains iridium on a support material consisting of silicon dioxide or zeolite. The catalyst of the invention is characterized by the fact that the iridium is present on the outer surface of the support material with an average particle size between 5 and 30 nm, preferably between 10 and 25 nm.
Surprisingly, this catalyst has an optimum rate of conversion for nitrogen oxides of more than 70% at very low exhaust gas temperatures of less than 350xc2x0 C. with an oxygen concentration in the exhaust gas of 8 vol. %. An oxygen concentration of 8 vol. % corresponds roughly to a normalized air/fuel ratio xcex in the exhaust gas of 1.5. Stoichiometric exhaust gas conditions are present at an oxygen concentration of about 0.7 vol. %.
An important factor relating to the catalyst according to the invention is that iridium is applied to a material which contains a high proportion of silicon dioxide as the support with a relatively coarse particle size of between 10 and 30 nm. Therefore, silicon dioxide itself or a dealuminized zeolite in the acid H-form is used as the support material. A ZSM-5 zeolite with a molar ratio (also called the modulus) of silicon dioxide to aluminum oxide of more than 20, preferably more than 100, is preferably used.
Zeolites are oxidic silicon/aluminum compounds with a specific crystal structure. They have the general composition
M2/nO.Al2O3.xSiO2.yH2O
wherein M represents a cation with the valency n and x is the modulus. The modulus is always greater than or equal to 2. The cations M are required to balance the charge in the zeolite lattice. They may be replaced by different ions by an ion exchange procedure. In this case, the new ion occupies the position of the ion being exchanged within the microporous structure of the zeolite. The number of ions which can be incorporated in the zeolite in this way is thus restricted by the ion exchange capacity.
Zeolites are often marketed in their Na+ or H+ form. The theoretical ion exchange capacity of a zeolite correlates directly with the number of anions in the lattice. To increase their hydrothermal stability, zeolites may be dealuminized using special techniques. Depending on the type of zeolite used, zeolites with moduluses of well above 100 may then result. However, the concentration of cations in the zeolite also decreases as the degree of dealuminization increases, since if the aluminum concentration is smaller, a smaller charge compensation effect is also required. Accordingly, the ion exchange capacity decreases drastically in dealuminized zeolites.